[robnolimit]

Boxing, X bracing and weight. The best weight-to-performance saying that I know is this: 100 lbs = 1/10th of a second. Now, this not an absolute, but if you have a 10 to 13 sec car, it's really close. On the flip side, Weight down low, below the Center of Gravity, will help the base handling. Also, the stronger chassis will help in traction, braking, handling and a big improvement in ride quality. So, in total gain, I would add 100 lbs of chassis stiffener weight with no thought about a negative to the weight.
So, yes, box the whole thing, but make sure it's square and level when you weld. Standard crossmembers make squares, or ladder frames. - not very strong. Try to make triangles. They are strong. We do sell the X member from our Pro-Tech chassis for $595 that can be fitted into the OE chassis. (not a 100% drop in, but not too bad to fit). I know that PB also has a pre fab X kit.